Telephone users often find it desirable to have information about the origin and/or destination of a received call prior to the call being answered. Some telephone communication systems now provide users with call origin information using a distinctive ringing signal which identifies whether the received call is an external (central office), an internal (intercom), or a priority call.
In other telephone communication systems, users are provided with call destination information using a personalized ringing signal. Personalized ringing enables each user to determine whether he or she is the intended recipient of an incoming call by listening to and decoding the ring signal. These distinctive and/or personalized ringing features are provided in communication systems using a coded ringing signal.
In one prior coded ringing circuit, each ringing code is comprised of a different pair of single frequency tones which are alternately enabled at a predetermined modulation rate during the ringing cycle. In such a circuit, however, the ability of the listener to discriminate between different frequency pairs limits the number of frequency pairs that can be utilized. Undesirably, this results in a small number of distinguishable ringing codes.